The embodiments disclosed herein relate to degradable balls, methods for their manufacture and methods for use in temporarily sealing perforations in wellbores, isolating segments of wellbores, and actuating wellbore tools.
Ball sealers are typically solid materials useful in sealing portions of a wellbore, subterranean formation, or both during completion operations. For example, ball sealers (sometimes referred to as perforation balls) may be used to seal a perforation in a wellbore to at least substantially reduce flow therethrough into the subterranean formation. In another example, ball sealers may be used in conjunction with a baffle disposed along and within the wellbore to at least substantially reduce flow through the baffle, thereby defining upper and lower segments of the wellbore.
Commercially available ball sealers are typically spherical with a hard, solid core made from nylon, phenolic, or aluminum. The solid cores may be covered with rubber to protect them from solvents and to enhance their sealing capabilities.
After the ball sealer has served its purpose, the ball sealer should be removed from the sealing location so as to return fluid flow between to the portion of the wellbore and/or formation that was previously blocked by the sealer. Typically, the density of the ball sealer is used to achieve removal. That is, ball sealers with a density greater than the fluids disposed within the wellbore (sometimes referred to as “sinkers”) may sink and accumulate at the bottom of the wellbore where they are out of the way of further operations. However, bottom hole accumulation can inhibit further wellbore operations. In other instances, ball sealers with a density less than the fluids disposed within the wellbore (sometimes referred to as “floaters”) may be flowed back to the surface and potentially reused. In either instance, this clean-up activity may be undesirable as it can delay further operations at the well and adds complications to the well treatment process. It is desirable to avoid either of these processes and would be desirable for the ball sealers to degrade downhole in such a manner as to not form undesirable products that may negatively affect any subsequent operations. More particularly, it is desirable that such balls degrade in a predictable manner, typically within a few hours or days.
Commercially available degradable ball sealers also suffer from a limited useful temperature range. For lower temperature applications, the degradable sealers are generally made from polyvinyl alcohol (“PVA”) and/or polyvinyl acetate (“PVAC”). For higher temperature applications, balls may be made from blends of polyethylene oxide (“PEO”), poly(propylene oxide) (“PPO”), and polylactic acid (“PLA”) (also referred to as polylactide). However, degradable ball sealers made from any of these materials may soften and deform in use, thereby losing their sealing capability. As such, a need exists for degradable ball sealers that exhibit the necessary rigidity when subjected to wellbore conditions to maintain sealing for the desired time interval, but that can degrade once the need for sealing has passed.